As the downsizing of a semiconductor device progresses, machining accuracy demanded to etching processing for a sample is becoming stricter. To perform highly accurate machining for a microscopic pattern on a wafer surface in a plasma processing apparatus, temperature management of the wafer surface during etching is important. However, because of demand for a larger size of wafer and an increase in etching rates, high frequency electric power applied to the plasma processing apparatus tend to increase, and particularly in etching of insulating films, large electric power of kilowatt order is begun to be applied. Impact energy of ions to the wafer surface as a result of application of large electric power increases, and an excessive temperature rise of the wafer during etching is becoming a problem. Because of demand for a higher level of shape accuracy, means for enabling fast and accurate control of wafer temperatures during process is demanded.
To control wafer surface temperatures within the plasma processing apparatus, the surface temperatures of an electrode with electrostatic chuck (hereinafter referred to as an electrode) contacting with the back surface of the wafer via a heat conduction medium need to be controlled. Conventional electrodes have controlled the surface temperatures of the electrodes by internally forming a passage of a refrigerant and passing a liquid refrigerant through the passage. The liquid refrigerant is supplied to the electrode passage after being adjusted to a target temperature by a cooling apparatus or a heating apparatus within a refrigerant supply apparatus. Since such a refrigerant supply apparatus temporarily pools the liquid refrigerant in a tank and sends out it after adjusting temperatures, and the heat capacity of the liquid refrigerant itself is large, this construction is effective to keep the surface temperature of the wafer constant. However, even if it uses such refrigerant supply apparatus, temperature response in the electrode is bad, fast temperature control is difficult, and heat exchange efficiency is low. Therefore, the apparatus has been enlarged in size accompanying a large quantity of input heat in recent years, and it has been difficult to control the temperatures of wafer surface to be optimum as required etching progresses.
Because of these facts, an apparatus that increases in-plane temperature uniformity of an electrode by adopting a refrigerant supply apparatus of direct expansion system (hereinafter referred to as a refrigerating cycle of direct expansion system) that provides an electrode with a compressor by which a refrigerant circulation system increases the pressure of a refrigerant, a condenser that condenses the pressurized refrigerant, and an expansion valve that expands the refrigerant, and evaporates the refrigerant within a refrigerant passage of the electrode for refrigeration is proposed in JP-A No. 089864/2005, and US 0081295A1/2007. In the refrigerating cycle of direct expansion system, since evaporation latent heat of a refrigerant is used, cooling effects are high, and evaporation temperatures of the refrigerant can be fast controlled by pressure. From the above, by adopting the direction expansion system as a refrigerant supply apparatus to the electrode, temperatures of semiconductor wafer during etching by a large quantity of input heat can be controlled highly efficiently and fast.
In the above JP-A No. 084/2008, a method for uniforming in-plane temperatures of a sample to be processed by providing a space to which a refrigerant is supplied, and a space in which the refrigerant evaporates, and successively enlarging the passage sectional area of the refrigerant space.
In conventional refrigerating cycles of direct expansion system proposed in the above mentioned JP-A No. 089864/2005, and US 0081295A1/2007, latent heat when a refrigerant evaporates from liquid to vapor is used for cooling, and the evaporation temperatures of the refrigerant can be controlled by pressure. If the pressure of the refrigerant is constant within the refrigerant passage of the electrode, evaporation temperature is also constant.
However, since the refrigerant flows through the passage while absorbing heat and evaporating, heat transfer rate change as phases change. Specifically, even when refrigerant pressure is kept constant within the refrigerant passage with in-plane temperature uniformity of the electrode in mind, heat transfer rate become uneven, and it is difficult to control the surface temperature of the electrode and wafer temperature to be uniform within plane. Furthermore, actually, refrigerant pressure also changes within the passage due to pressure loss of the refrigerant. Pressure loss per unit length (hereinafter referred to as pressure loss) that occurs within the passage changes with phase change of the refrigerant. Therefore, adopting a refrigerating cycles of direct expansion system as a refrigerating mechanism of an electrode poses a technical problem of temperature distribution uniformity control within a plane.
The above JP-A No. 089864/2005 does not take a change in heat transfer rate accompanying such a phase change into account. The above US 0081295A1/2007 discloses technology that disposes an accumulator between a refrigerant ejection port of an electrode and a compressor and connects a bypass loop to a refrigerant supply port and the refrigerant ejection port to uniform in-plane temperature distributions of the electrode. The technology of the above US 0081295A1/2007 keeps the inside of a refrigerant passage of the electrode in a vapor-liquid mixed state and uniforms temperature distributions by controlling the opening of an expansion valve and a bypass flow rate for the expansion valve. As an example, liquid to vapor in the refrigerant supply port of the electrode is controlled to 40 to 60%, and liquid to vapor in the refrigerant ejection port is controlled to 10%. However, also in the above US 0081295A1/2007, a change in heat transfer rate accompanying phase change within the refrigerant passage of the electrode is not taken into consideration.
Also in JP-A No. 34408/2008, a change in heat transfer rate accompanying phase change is not taken into consideration. A refrigerant passage within a sample stand proposed in JP-A No. 34408/2008 has a problem that a passage connecting a refrigerant supply space and a refrigerant evaporation space has a contraction structure, and since pressure after contraction decreases, a pressure setting range of the refrigerant evaporation space is limited to a low pressure and the temperature setting range of a sample to be processed becomes narrow.
The inventors, as means for solving the above-described problem, previously proposed a method for changing the sectional area of a passage according to a dryness degree of a refrigerant from the entrance to the exit of a refrigerant passage, based on a property affording the relationship between dryness degrees of the refrigerant and heat transfer rate so that heat transfer rate of the refrigerant in the refrigerant passage within an electrode become uniform within a plane (Japanese Patent Application No. 016881/2007, filed on Jan. 26, 2007, and corresponding U.S. patent application Ser. No. 11/676,593, filed on Feb. 20, 2007, hereinafter referred to as a prior filed invention). Specifically, according to the prior filed invention, in general properties of constant passage sectional areas, in places where the heat transfer rate of a refrigerant is large, the heat transfer rate of the refrigerant is reduced by enlarging a passage sectional area to decrease the flow rate of the refrigerant. Conversely, in places where the heat transfer rate of the refrigerant is small, the heat transfer rate of the refrigerant is reduced by reducing a passage sectional area to increase the flow rate of the refrigerant. In this way, from the entrance to the exit of the refrigerant passage within the electrode, the values of heat transfer rate can be made flat. In the prior filed invention, as a concrete example, a refrigerant passage includes at least three passage regions connected successively in cascade from a passage regions is larger than that of other passage regions.
According to the prior filed invention, since the values of the heat transfer rate can be made flat from the entrance to the exit of the refrigerant passage within the electrode, the problems of the above JP-A No. 089864/2005, US 0081295A1/2007, and Japanese Patent Application No. 016881/2007 are significantly solved.
As a result of inventors' research, it was found that when heat inputted to a sample to be processed is almost the same as the capacity of a refrigerating cycle in a plasma processing apparatus, and a refrigerant can be evaporated to a dryout region within a refrigerant passage, the construction and the temperature control method of the prior filed invention are significantly effective. On the other hand, it was found that, in the case of a plasma processing apparatus in which an input heat quantity from plasma is large, the construction and the temperature control method of the prior filed invention may be insufficient. Specifically, in a plasma processing apparatus in which an input heat quantity from plasma is large, the capacity of a required refrigerating cycle is large. In a refrigerating cycle of large capacity, since the flow rate of a refrigerant circulating within a refrigerating cycle increases, the property of pressure loss must be taken into account. In this case, if a sectional area is reduced against dryout of the refrigerant near the exit of the refrigerant passage, pressure loss increases in a reduced region of a sectional area and evaporation temperature itself of the refrigerant may change.
Therefore, to the temperature of a wafer on an electrode highly efficiently, fast, and uniformly within a plane using a refrigerating cycle of direct expansion system, a study of electrode structure showing further improvement of the prior filed invention was required.